(1) Field of the Invention
The present invention relates to a transmitter and a transmission method. The present invention relates to a technique suitable for use in radio communication techniques such as mobile communication systems that use a DFT-S-OFDM (Discrete Fourier Transform-Spread-Orthogonal Frequency Division Multiplexing) scheme, for example.
(2) Description of Related Art
At present, in 3GPP (3rd Generation Partnership Project), standardization of enhancement (Long Term Evolution) of third generation mobile communication systems is under consideration. For an uplink access scheme, adoption of SC-FDMA (Single Carrier-Frequency Division Multiple Access) is considered and a method of implementing the SC-FDMA using the DFT-S-OFDM is one of the promising approaches.
FIG. 8 shows a configuration that focuses attention on an essential part of a transmitter that adopts the DFT-S-OFDM. The transmitter shown in FIG. 8 includes, for example, a modulator 101, a DFT (Discrete Fourier Transformer) 102, a subcarrier mapper 103, an IFFT (Inverse Fast Fourier Transformer) 104, and a CP (Cyclic Prefix) inserter 105. The following describes, as an example, parameters for the case in which the transmission frequency band is 5 MHz.
In the transmitter having the above-described configuration, a transmission data signal is modulated by the modulator 101 by a required modulation scheme such as QPSK or 16 QAM. The modulated data signal has a symbol rate of 4.5 MHz.
The modulated data signal is subjected to a DFT processing for every 300 symbols (which is called a block) in the NDFT (e.g., 300)-point DFT 102, whereby the signal is converted from a time-domain signal to a frequency-domain signal. That is, the DFT-processed modulated data signal is converted to a signal composed of NDFT (=300) subcarriers.
The subcarrier mapper 103 maps the 300-subcarrier signal onto an NIFFT (e.g., 512)-point IFFT (IFFT 104), thereby performing a frequency-domain arrangement. Depending on the subcarrier mapping method, the transmission bandwidth is extended in form and thus this subcarrier mapping is called a spread.
The subcarrier-mapped signal is subjected to an IFFT processing in the IFFT 104, whereby the signal is converted from the frequency-domain signal to a time-domain signal again. An example of FIG. 8 shows an operation in which an output from the 300-point DFT 102 is continuously arranged on the 512-point IFFT and a zero signal is inserted in the remaining 212 points.
By this, the signal of 300 points per block prior to the DFT is converted to a signal of 512 points per block after the IFFT. That is, the signal of 4.5 MHz prior to the DFT is over sampled to 7.68 MHz after the IFFT.
To every block of the IFFT-processed signal, a CP (Cyclic Prefix) is added by the CP inserter 105. Gathering, for example, seven CP-added blocks composes one frame. An example of the frame is shown in FIG. 9. In this case, two blocks (see hatched portions) in one frame are allocated as pilot bocks for pilot symbols. Note that the pilot symbols (hereinafter also simply refereed to as “pilots”) are signals known by a receiving end and are used for channel estimation for demodulation of a data channel, for example.
Meanwhile, reasons that the DFT-S-OFDM scheme is considered in the 3GPP include an improvement in frequency utilization efficiency. In third generation mobile communication schemes (hereinafter also referred to as the “3G schemes”) such as W-CDMA, for a band limitation method, time-domain FIR (Finite Impulse Response) filter is used. With this method, it is difficult to perform steep band limitation and thus there is a need to perform band limitation by an FIR filter with a high roll-off rate. For example, in the 3G schemes, the symbol rate of a signal to be transmitted in a bandwidth of 5 MHz is limited to 3.84 MHz.
On the other hand, in the DFT-S-OFDM, as described above, a DFT-processed frequency-domain signal is converted to a time-domain signal using an IFFT which is larger in size than the DFT, whereby over sampling and band limitation are performed at the same time. In the configuration in FIG. 8, waveform shaping is not performed and thus it is equivalent to performing band limitation using a window function (rectangular filter) with the roll-off rate α being 0; however, as shown in FIG. 10, for example, it is also possible to perform gradual band limitation using a waveform shaping filter 106 with the roll-off rate being α>0. In an example of FIG. 10, in the case where it is assumed that the transmission bandwidth is 5 MHz, the symbol rate is lowered to 4.08 MHz and the DFT size is reduced to 272. As such, by allowing a signal which is DFT-processed by the DFT 102 to pass through the waveform shaping filter 106, gradual band limitation is performed.
An operation of a waveform shaping processing by the waveform shaping filter 106 is shown in FIG. 11. In FIG. 11, part of a DFT-processed 272-subcarrier signal (14 subcarriers at each end) is cyclically copied, whereby a signal of 300 subcarriers in total is generated. Then, by multiplying the signal by a coefficient of a raised cosine function with the roll-off rate α being 0.1, waveform shaping is performed.
As such, in the DFT-S-OFDM, merely by performing multiplication by a coefficient in a frequency-domain, waveform shaping can be performed and thus the roll-off rate α can be relatively easily changed (controlled).
Advantages of performing gradual band limitation by increasing the roll-off rate α include an effect of reduction of a PAPR (Peak to Average Power Ratio). For example, as shown in FIGS. 10 and 11, in the case where band limitation with the roll-off rate α being 0.1 is performed, the PAPR is reduced by the order of 0.5 dB as compared with the case where band limitation is performed by a rectangular filter with the roll-off rate α being 0, as shown in FIG. 8. In an uplink where transmission is performed from a terminal, by reducing the PAPR, the maximum transmission power can be increased and thus advantages such as an increase in radio wave reaching distance and an improvement in amplifier efficiency can be obtained. As such, the frequency utilization efficiency and the PAPR have a trade-off relationship.
In the DFT-S-OFDM, a DFT computation processing is required and thus generally there is a problem of an increase in circuit size. If an FFT whose number of points is a power of two can be used for a DFT, a significant reduction in circuit size is achieved; however, in practice, since parameters are designed to obtain optimum frequency utilization efficiency and PAPR characteristics, the DFT size is not always a power of two. Thus, in the DFT-S-OFDM scheme, there is a tendency that the circuit size increases.
The following Non-Patent Documents 1, 2, and 3 relate to the DFT-S-OFDM scheme which is under consideration in the 3GPP. The Non-Patent Document 1 is a document prepared at an early stage when the DFT-S-OFDM system is proposed in the 3GPP and describes basic matters regarding the DFT-S-OFDM technique. The Non-Patent Document 2 introduces a band limitation method and results of consideration of PAPR characteristics in the DFT-S-OFDM scheme. The Non-Patent Document 3 proposes adaptive control of the roll-off rate and the size of a DFT.
[Non-Patent Document 1] 3GPP, R1-050584, Motorola, “EUTRA Uplink Numerology and Design”, Jun. 20-21, 2005
[Non-Patent Document 2] 3GPP, R1-050702, NTT DoCoMo, et al., “DFT-Spread OFDM with Pulse Shaping Filter in Frequency Domain in Evolved UTRA Uplink”, Aug. 29-Sep. 2, 2005
[Non-Patent Document 3] 3GPP, R1-060993, NTT DoCoMo, et al., “Investigation on Adaptive Control of Roll-off Factor for DFT-Spread OFDM Based SC-FDMA in Uplink”, Mar. 27-31, 2006
As described above, in the DFT-S-OFDM scheme, the PAPR and the frequency utilization efficiency have a trade-off relationship. The PAPR characteristics are known to greatly depend also on a modulation scheme. For example, in 16 QAM, the PAPR is higher than QPSK by the order of 1.0 dB.
Hence, in the case where the modulation scheme for transmission data frequently changes depending on adaptive modulation, the PAPR significantly changes and thus there is a need to perform control to suppress the PAPR to a level lower than a certain level. For example, a method may be considered in which for 16 QAM-modulated data, the PAPR is actively reduced, and for QPSK-modulated data, the frequency utilization efficiency is actively improved rather than reducing the PAPR, whereby the transmission efficiency of the entire system is improved.
Specifically, as proposed also by the Non-Patent Document 3, in the case where the modulation scheme is 16 QAM, to actively reduce the PAPR, the roll-off rate α is increased and the DFT size is reduced (e.g., as shown in FIGS. 10 and 11, the roll-off rate is set to 0.1 and the DFT size is set to 272). On the other hand, in the case where the modulation scheme is QPSK, there is no need to further reduce the PAPR and thus the roll-off rate α is reduced and the DFT size is increased (e.g., as shown in FIG. 8, the roll-off rate α is set to 0 and the DFT size is set to 300).
As such, in the DFT-S-OFDM scheme, by changing the roll-off rate α and the DFT size according to the modulation scheme, data transmission efficiency can be improved.
However, as described above, there is a problem that the circuit size of a DFT is large as compared with that of an FFT. Thus, in the case where the DFT size is changed according to the modulation scheme, a plurality of types of DFT circuits with a large size need to be provided on hardware, further increasing the impact of circuit size.